In commonly assigned Hamilton U.S. patent application Ser. No. 371,189, filed May 29, 1964, now U.S. Pat. No. 3,446,856, there is disclosed and claimed a method for methylating the ortho positions of phenol by the vapor phase reaction of a phenol with methanol in the presence of magnesium oxide as a catalyst at a catalyst bed temperature in the range of 475.degree. to 600.degree.C. Under the conditions described in the Hamilton patent, phenol is selectively ortho-methylated in yields of 95%. Thus, the reaction offers a means for economically converting phenol to ortho-cresol, a useful disinfectant and wood preservative and for converting both phenol and ortho-cresol to 2,6-xylenol, a monomer which can be polymerized to form poly(2,6-dimethyl-1,4-phenylene)oxide, a high performance thermoplastic material.
While the Hamilton patent provides an economic synthesis for both 2,6-xylenol and ortho-cresol from phenol, the service life of the magnesium oxide catalyst is relatively short due to the high temperature at which the reaction is required to take place, i.e., about 90 to 100 hours service life at the typical reaction temperature of about 530.degree.C. Also, the magnesium oxide catalyst of the Hamilton patent is only moderately selective with respect to methanol, with methanol selectivity being in the range of about 40 to 50%. This means more than about two moles of methanol are required for each mole of methanol entering into the reaction with phenol. In addition, the use of unmodified magnesium oxide in powdered form or in the form of a weakly sintered composite results in a rather large induction period for maximum selectivity. The term "induction period" may be defined as the period from the time of starting the reaction to the time at which the catalyst reaches and maintains maximum ortho-alkylation selectivity. Finally, the use of magnesium oxide in powdered or sintered form provides various processing difficulties.
Many of the above-noted difficulties are overcome by the process of the said copending U.S. patent application Ser. No. 717,919, filed Apr. 1, 1968. Therein, a process is disclosed for the ortho-alkylation of phenol comprising the reaction of a phenol having at least one unsubstituted ortho position with an alcohol in the presence of a catalyst consisting of a mixture of magnesium oxide and manganese sulfate.
The catalyst of copending patent application Ser. No. 717,919 is prepared by pulverizing the magnesium oxide and wet-mixing the magnesium oxide with manganese sulfate and water or, alternatively, by impregnating the pulverized magnesium oxide with an aqueous solution of manganese sulfate. Preferably, the manganese sulfate constitutes from 2 to 10% of the catalyst on a dry solids basis. After mixing, it is preferred to mold the catalyst to a desired shape. It is disclosed that interaction between the manganese salt and the magnesium oxide upon contact with water from wet mixing or impregnation with aqueous solution is accompanied by the evolution of heat. Since the surface to weight ratio of the magnesium oxide is very high, for example, of the order of 180 square meters per gram, the heat brings about a noticeable increase in the mixture temperature under normal impregnating or mixing conditions. This appears to result in the formation of a lattice of enhanced catalytic activity containing the magnesium oxide and manganese sulfate.
With the catalyst of application Ser. No. 717,919, selectivity favoring ortho-methylation over meta- or para-methylation is the same as that of the Hamilton patent at temperatures as low as 440.degree.C. It is advantageous to operate at temperatures as low as 440.degree.C. instead of, for example, 530.degree.C., because catalyst life is extended considerably before regeneration or other treatment is needed. Such catalysts have been operated without reduction in catalytic activity for periods in excess of 800 hours, in contrast to a maximum life of 90-100 hours for the Hamilton catalysts. In addition, the magnesium oxide-manganese sulfate catalyst is superior because of the increased strength imparted to the catalyst particles by the manganese oxide lattice formed by the reaction between manganese sulfate and magnesium oxide in the presence of water. Therefore, there is much less tendency for particles of catalyst to flake off in operation or from handling and the service life is thus extended with a minimum loss of catalyst.
Although the catalyst of the said copending application Ser. No. 717,919 provides the advantages noted above, there is observed an undesirable increase in the induction period required for maximum reaction selectivity. This means that about 24-100 hours is required to reach a molar phenol selectivity level of over 90%. During the induction period, a relatively large amount of undesired 2,4,6-mesitol is produced. It is believed that the presence of the sulfate ion in the catalyst is responsible for the longer induction period.
It has now been found that magnesium oxide mixed or bonded with manganese oxide or a mixture of manganese oxides has the advantages of the catalyst of the above-noted copending application and also, because sulfate is eliminated, has the further advantage of having a reduced induction period for maximum ortho-alkylation selectivity. Thus, the present invention contemplates an ortho-alkylation catalyst consisting of magnesium oxide mixed with manganese oxides, substantially free of sulfate, permitting the ortho-alkylation reaction to proceed with a high degree of both methanol and phenol selectivity and with a reduced induction period for maximum selectivity. Moreover, the new catalysts may be molded to any desired shape and will have strength properties sufficient to prevent particles of the the catalyst from breaking or flaking off in operation or handling, thus substantially extending the service life of the catalyst with a minimum of loss during operation. In addition the optimum reaction temperature may be reduced from 500.degree.-540.degree.C. for conventional catalysts down to 465.degree.-485.degree.C., thereby improving the overall economy of the process and increasing the catalyst life further.
It has also been discovered that if the catalysts according to this invention are treated with methanol vapor before use, there is an unexpected, significant increase in conversion of phenols to ortho-alkylated products. This in-situ activation provides an economical means of heating up a large commercial size reactor and results in a significant increase in production rate, of at least about 50% and more.
Accordingly, a primary object of the present invention is to provide an improved catalyst for the ortho-alkylation of a phenol with an alcohol in a highly selective manner and in high yield.
Another object of the invention is to provide a magnesium oxide catalyst having superior physical strength properties, which may be molded to any desired shape and which will have a service life of many hundreds of hours before needing regeneration or other treatment.
Still another object of the invention is to provide a process for formation of ortho-alklated phenols where the induction period required to reach maximum selectivity is low.
A further object of the invention is to provide an activated catalyst by exposure to methanol vapors.